The present invention relates to shunt-connected thyristor-controlled capacitors (STCC) where a capacitor is connected in shunt across a substantially inductive transmission line using thyristor type switches, and more particularly, to vernier type control of the STCC to vary the effective reactance of the capacitor.
Reactive compensators such as static VAR compensators (SVC) are used to control voltage and provide reactive power in AC power transmission systems and have many practical applications in those systems. They are used to maintain voltage at or near a constant level under slowly varying conditions in response to load changes, to correct voltage changes caused by unexpected events (e.g. load rejections, generator and line outages), and to reduce voltage flicker caused by rapidly fluctuating loads. They also are used to improve power system stability and power factor as well as current phase imbalance. Controlled switching of reactive elements regulates the amount of capacitive/inductive loading applied to the AC transmission line thereby modifying the reactive power delivered by the power generation system.
Volumes have been written regarding the control of thyristor switched capacitors (TSC). Until recently, this work has focused solely on determining when to turn the thyristor valve either fully on or fully off. Thus, only two reactances are provided. Exemplary works in this area include Static Compensators For Reactive Power Control, Canadian Electrical Association, edited by R. M. Mathur, Cantex Publications, Winipeg, 1984, and "Static VAR Compensators," edited by I. A. Erinmez, CIGRE Report by WG 38-01TF2, Paris, France.
Recently, it was suggested by J. J. Vithayathil and others in a 1988 paper entitled "Case Studies Of Conventional And Novel Methods Of Reactive Power Control On An AC Transmission System" and in U.S. Pat. No. 5,032,738 that by using partial conduction of an inductor across a series capacitor, the effective reactive compensation of the series capacitor could be varied. However, the inductor is controlled in the same way that a thyristor-controlled reactor (TCR) of a static VAR compensator is controlled, i.e. based on timing signals derived from zero crossing times of the capacitor voltage.
Even more recently, there has been an effort towards designing a thyristor-controlled capacitor for series reactive power compensation that permits partial conduction (sometimes referred to as "vernier" control) of the thyristor valve to achieve an effective admittance other than those values associated with fully on or fully blocked thyristor valve operation. See for example U.S. Pat. No. 5,202,538 issued Apr. 13, 1993 the disclosure of which is incorporated herein by reference. Related U.S. patent application Ser. No. 08/048,133 filed on Apr. 13, 1993 and entitled "Transient Damping Thyristor Controlled Series Capacitor System" describes firing control logic for a series thyristor-controlled capacitor for increased capacitive reactance and certain firing to damp transients in the inductive region, the disclosure of which is incorporated herein by reference.
Thus, while partial-conduction control has been implemented for a thyristor-controlled reactor in parallel with series capacitors, such control of a shunt-connected capacitor has not been considered possible.
Accordingly, the present invention provides a basic firing control scheme, including a method and apparatus, for vernier operation of a shunt-connected thyristor-controlled capacitor (STCC) using phase controlled firing of the thyristor valve based on measured source voltage, measured thyristor current, and a desired predetermined thyristor current. The STCC includes a capacitor and surge inductor connected in shunt across the AC transmission line via a bidirectional (antiparallel) pair of thyristors, i.e. the thyristor valve. Vernier control is achieved by predicting an upcoming firing angle that switches the thyristors so as to partially conduct current through the capacitor and thyristors. In other words, by selectively varying the firing advance angle to angles other than those corresponding to fully conducting and fully blocked STCC operation, the thyristors provide a varying admittance. Thus, in response to a command from a higher level controller to change the reactive power, e.g. the thyristor current, from its present value to a new desired value, the present invention determines a new firing angle to partially conduct current through the thyristors and provide the desired amount of inductive or capacitive reactance.